One of those minority groups, the Yazidi, found themselves cornered by ISIS fighters on a mountain near their ancestral homeland of Sinjar. Thousands of Yazidi men, women, and children were trapped, rapidly running out of food and water but unable to descend from the mountain for fear of the militants below.

The Yazidis are hardly a household name for most Americans, but they have lived in the same region in northern Iraq for centuries, with communities in Syria and Turkey as well. They're small in number - estimates range from a tens of thousands to half a million - and they're an insular group, keeping mostly to themselves even as the borders and political structures around them have gone through often-bloody realignments.

In 1892, the Ottomans launched a campaign to forcibly convert the Yazidis. They were ultimately rebuffed, but not before they massacred thousands, captured a Yazidi shrine, and forced a Yazidi prince to convert to Islam.

"You see flavors of many things that we find familiar, but it is their own practice. It's their own faith," explained Farah Pandith, the former special representative to Muslim communities under former Secretary of State Hillary Clinton and the current secretary, John Kerry, in an interview with CBS News. "It's what they've been doing for thousands of years."

One central figure in the Yazidi faith is a fallen angel named "Malak Taus," whom the Yazidi also refer to as "Shaytan."

"Shaytan" also happens to be the Arabic word for "devil." As a result, some Muslims, including many of the Sunni militants fighting with ISIS, mistakenly believe the Yazidis are "devil worshipers."

"They've been accused of this since the 16th and 17th centuries," when Muslim clerics issued the first fatwa (a religious proclamation) against the Yazidis, Allison told CBS News.

She blamed the misunderstanding on widespread "confusion" about the role of the fallen angel in the Yazidis' faith.

The fallen angel, also called the "peacock angel," is one of seven angels venerated by the Yazidis. Because they worship the peacock angel, and because the peacock is associated with Satan in some strains of Islam, Allison said, the Yazidis have been occasionally branded "devil worshipers."

"But it's not devil worship in any way we would recognize," she said. "The Yazidis themselves don't in any way worship him as an evil figure who has to be appeased."

For all the importance attached to religious disputes, Pandith emphasized, a more fundamental intolerance may be at play behind ISIS' campaign to conquer and convert.

"We've heard a lot of conversation about the 'fallen angel' character," she explained. But for ISIS militants hell-bent on "decimating anything that does not resemble what they believe," she added, any excuse for a purge will suffice.

"Right now, it's the Yazidis. Tomorrow it will be something else," she said. "It really is about eradicating anything and everyone that does not resemble their ideology."